This invention relates to automatic transmissions having a layshaft kinematic arrangement, particularly to automatic transmissions having dual input clutches, but no torque converter.
Dual clutch layshaft transmissions are essentially two automated manual transmissions in a common housing, one providing odd numbered gears and the other providing even numbered gears. Shifts between odd and even numbered gears can be accomplished without interrupting power flow. While operating in an odd numbered gear, couplers can be actuated to configure the transmission for the desired even numbered gear. Then, power is transferred to the even numbered gear by engaging the even clutch while disengaging the odd clutch in a coordinated fashion.
In a front wheel drive vehicle, the axial space available for the transmission is limited by the width of the engine compartment and the length of the engine. For this reason, dual clutch transmissions typically use at least two countershafts so that components can be placed side by side instead of along the main transmission axis. Some arrangements, such as those described in U.S. Pat. Nos. 7,044,014 and 7,077,025, utilize more than two countershafts to achieve very short lengths.
A well known method of reducing the length of a two countershaft transmission is having a single pinion on an input shaft drive gears on both countershafts. This reduces the overall length of the transmission by the face width of a gear. A disadvantage of this method is that it reduces the ability to adjust speed ratios by selecting the size of each gear, because a change in the size of the pinion forces a change in the size of both driven gears. This disadvantage is partially alleviated by the fact that there are two final drive ratios which can be adjusted separately to achieve the desired ratio spacing. However, when more than one input pinion is re-used for two gears, the number of degrees of freedom for adjusting ratios is less than the number of ratios. As a result, a designer is forced to accept some ratios that are larger or smaller than desired.
It is desirable to have the speed ratio for reverse be about the same as the speed ratio for first gear, since both are used to move the vehicle from rest. However, in a typical layshaft transmission, it is difficult to obtain a reverse speed ratio this high with a single idler gear. The size of the pinion is limited and the size of the driven gear cannot be as large as the driven gear for first because the teeth must clear the teeth on the pinion. As a result, a stepped pinion is sometimes used to provide an additional opportunity to multiply the torque. However, stepped pinions increase the required axial length.